A typical active shunt circuit generally uses gain and capacitance to produce a virtual-impedance that is the actual resistance reduced by the gain. FIG. 1 is a circuit diagram illustrating an example of a conventional active shunt circuit 100. In the example, the active shunt circuit 100 uses gain G1(s) and capacitance C0 to produce a virtual impedance ZIN that is the actual resistance R0 reduced by the gain (α).
FIG. 2 is a circuit diagram illustrating a first example of a conventional two-range auto-ranging source-measure unit (SMU) circuit 200. The SMU circuit 200 includes a voltage source V-DAC, a current source I-DAC, and an output 220 that is configured to be electrically coupled with a device under test (DUT).
The SMU circuit 200 also includes a power-providing circuit 250 that has a first amplifier 206 and a second amplifier 208 that is electrically coupled between the DUT and either the current source I-DAC or a buffer 210 dependent upon the present position of a first switch S1. The power-providing circuit 250 also has first and second gain stage amplifiers 212, 214 that are electrically coupled between the DUT and either the voltage source V-DAC or the buffer 210 dependent upon the present position of a second switch S2. A first resistor R0 is electrically coupled with the DUT and the buffer 210. A second resistor R1 is electrically coupled between the first resistor R0 and the second resistor R1.
In the example, the SMU circuit 200 sources voltage across the DUT when the two switches S1 and S2 are in the down positions as shown in the figure. The SMU circuit 200 has a control loop with an interaction between the DUT and the current sensing resistor that may be determined by the following:β=ZDUT+RS/ZDUT 
where RS is either the first resistor R0 or the second resistor R1 depending on which range is active.
In situations where the impedance of the DUT (ZDUT) is smaller than the active range resistance RS, β becomes significantly less than one and the control loop undesirably slows down.
FIG. 3 is a circuit diagram illustrating a second example of a conventional two-range auto-ranging source-measure unit (SMU) circuit 300. In the example, the SMU circuit 300 uses a single control loop and switch S1 to transition between a voltage source V-DAC (i.e., when the switch S1 is in the down position as shown in the figure) and a current source I-DAC (i.e., when the switch S1 is in the up position).
In this second example, the resistors R0 and R1 are arranged in parallel. This is in contrast to the SMU circuit 200 of the first example, in which the resistors R0 and R1 are in series.